on the possibility to use a hybrid synchronous machine as
TRANSCRIPT
On The Possibility To Use A Hybrid Synchronous Machine As An Integrated Starter-Generator
I.A. VIOREL – R. MUNTEANU – D. FODOREAN – L. SZABÓ
Technical University of Cluj RO-400020 Cluj, Daicoviciu 15, Romania
Phone: +40-264-401-232 / Fax: +40-264-594-921 e-mail: [email protected]
Abstract-The hybrid and mild hybrid cars require high
performance integrated starter-generators (ISG). Several electrical machines were designed and used as ISGs up today. A hybrid synchronous machine (HSM) to be employed for such purpose is proposed in this paper. The HSM has the excitation field produced both by permanent magnets and auxiliary excitation winding. Three construction variants of the HSM were studied comparatively. A prototype was designed, constructed and tested. The background of the HSM, operating as a generator is discussed, too. All the obtained results, by tests or by computer simulations, emphasise that HSM could be an excellent ISG for a hybrid car.
I. INTRODUCTION
The hybrid and mild hybrid cars require an integrated starter generator (ISG), which can also participate, in certain conditions, to the motoring process [1, 2]. Such an ISG should fulfil some important requirements as [3]: − High torque density − High starting torque − High speed range when acting as a motor − Almost constant output voltage at a large speed domain − Overload capability for short period of time − Simple and robust construction. Different types of electric machines were studied as possible
ISG, from the conventional squirrel cage induction machine to the unconventional switching reluctance or transverse flux machines [4, 5, 6]. Among these variants also the hybrid synchronous machine (HSM) can be considered.
Basically the hybrid synchronous machine is a permanent magnet one, but has an additional field winding supplied or not function of the necessities. This additional field winding can strength or weaken the excitation field, depending on the supply polarity and consequently on its current direction [7]. Both actions are necessary in motoring or generating operating mode of HSM, due to the above presented requirements.
When motoring HSM requires the strengthening of excitation field at its start and in low speed heavy duty operation, and respectively the field weakening in order to achieve an extended speed domain. Acting as a generator HSM requires an excitation field control, which contributes, together with the electronic converter control, to maintain the dc output voltage constant for entire drive speed domain.
HSM offers a good figure as far as the power, or torque density is concerned, and the quite complicated construction
and a bit higher cost is fully compensated by its performances obtained in both regimes.
The aim of the paper is to analyze the possibility to use a HSM as an ISG. Different variants for ISG construction are analysed, in all considering the series field flux pattern, which is more adequate for such an application. The variant with the both field sources on the rotor was found the best one. In such a structure a prototype was designed and constructed mainly to check the calculated results, the design procedure developed and the assumptions made. Some tests and some simulations are presented. All results stand by to sustain the fact that HSM can be a real competitor on the ISG market.
II. HYBRID SYNCHRONOUS MACHINE CONSTRUCTION
Three different variants of the HSM to be applied as ISG were studied, all in a structure in which the two field fluxes are in series [8].
The first variant with surface permanent magnets (SPM) and the additional field winding on exterior rotor is given in Fig. 1.
The surface mounted permanent magnets are placed on the inner part of the exterior rotor. The field winding is wound in
a) cross-section
b) flux pattern
Fig. 1. All field sources on the rotor HSM variant.
1-4244-0726-5/06/$20.00 '2006 IEEE 1195
the slots of the rotor. The flux-weakening is obtained supplying de field winding in a manner as to create an opposite flux to the permanent magnet flux. Similarly, by changing the current direction in the additional winding a flux-strengthening can be obtained. The stator and rotor armatures have to be manufactured of very good quality laminations in order to minimize the effects of saturation. The flux weakening also means reducing the iron losses.
In order to avoid the slip rings in the rotor in the case of the second variant in study the field winding was moved on the interior stator of the HSM (see Fig. 2). In this case the flux changing is performed exactly in the same manner as it was presented previously.
The third HSM structure considered is similar to the second one, with the single difference that the rotor has interior permanent magnets (IPM) in a flux concentrated pattern. Because the performances of this variant depends strongly on the rotor number of poles, several cases were taken into study in the range of 4÷16 poles.
From the numerous results obtained, under a 2D finite element method (FEM) analysis done on the HSM variants, with the same main dimensions, only three shaded colour plots of the magnetic flux densities in the considered variants, which are quite representative, are given in Fig. 4.
To compare the three HSM variants their main characteristics were computed analytically and by means of FEM analysis.
The efficiency, the power density, the active parts weight / costs ratio, the permanent magnet's weight, the flux weakening capabilities and the existence of the sliding contact were the main analysed characteristics. In the case of the variant with IPM rotor and flux concentrating topology, the best structure, that having 8 pole pairs, was taken into account.
The main results of this comparative study are given in Table I.
a) cross-section
b) flux pattern
Fig. 2. HSM variant with SPM rotor and additional field winding on stator.
Fig. 3. HSM variant with IPM rotor and additional field winding on stator.
a) first variant
b) second variant
c) third variant
Fig. 4. The shaded colour plots of the magnetic flux densities.
1196
As it can be seen from the Table 1, the first variant has the best performances regarding almost all the characteristics The single drawback of this HSM variant is the existence of the slip rings-brushes system.
Taking into account the above study on the performances of the three HSM variants, respectively the possibilities to construct the machine, the first variant was chosen and an inner rotor construction prototype resulted. The constructed prototype is presented further.
III. HSM IN A GENERATING REGIME
ISG is operating mainly as a generator to supply the consumers and to charge the battery. Therefore here some theoretical developments concerning the hybrid synchronous generator (HSG) are presented.
The HSG mathematical model, considering, for the sake of generality, the rotor excitation fields on both d- and q-axis, is [9]:
fqqqq
fdddd
dqq
qdd
iL
iL
Riv
Riv
λλ
λλ
ωλ
ωλ
+=
+=
−=+
=+
(1)
or, with general state vectors,
fqfdqqddqds
sss
jijLiLj
jiRv
λλλλλ
λω
+++=+=
−=+ (2)
The motoring regime mathematical model can be obtained from the generator’s one by simply considering the change of the power circulation from the source to the machine, when for generator it is on the opposite direction [10].
The vectors’ diagram in generator regime is given in Fig. 5. The notations are the usual ones, − vd, vq; id, iq; λd, λq; Ld; Lq; are d and q-axis voltages,
currents, flux linkages and synchronous inductances respectively.
− ω is rotor speed, equal with the stator emf pulsation for one pole pair machine.
The induced emfs are [11]:
)(0
0
fqq
fdd
jjE
jE
λω
ωλ
−=
−= (3)
Neglecting the stator phase resistance the scalar equations are:
)cos()sin(
sincos
0
0
000
00
00
ϕγϕγ
θαγ
ωω
γγ
+=+=
+=
==
−=+=
sq
sd
dd
qqsq
ddsd
IIII
LXLX
IXVEIXVE
(4)
A general case of a HSG operating on a symmetrical load with an electronically controlled (EC) LC block on each phase for voltage control is considered, Fig. 6.
The phase currents, and impedance, are:
22
11
⎟⎟⎠
⎞⎜⎜⎝
⎛++⎟⎟
⎠
⎞⎜⎜⎝
⎛+=
=
⎟⎟⎠
⎞⎜⎜⎝
⎛+=
+=
B
LB
L
BL
L
BL
B
LB
BLss
LBss
LBs
ZZX
ZZX
ZZR
ZZRZ
ZZZVI
ZZVI
III
(5)
TABLE I MAIN PARAMETERS OF THE HSM VARIANTS
HMS variant Item
1 2 3
Efficiency 88% 83% 78%
Output power / weight 5.5 kW / 28.2 kg 5.5 kW / 36.8 kg 5.5 kW / 95 kg
Weight / costs of active parts 28.2 kg / 423 € 36.8 kg / 641 € 95 kg / 1400 €
Magnet's weight 1.8 kg 3.6 kg 15.3 kg
Flux weakening capability Good Medium Weak
Sliding contact Yes No No
idλfdd
q
LdidLqjiq
jλfq λS
E0d
E0q
E0
jiqiS
φθ0
γ0
α0
jXdid
jXq(jiq)
RiS
vS≈-jωλS
Fig. 5. Vectors’ diagram, HSG steady-state regime.
Fig. 6. HSG load and EC block on one phase.
1197
where RL, RB, XL, XB, ZL, ZB are load, respectively LC block resistances, reactances and impedances.
After some simple mathematical calculations results:
qBL
s
qdqd
qBL
s
dqs
XZZ
ZV
EXXE
XZZ
ZV
XXV
⋅
−+
+
⋅
−=
0000
0
cossin/
cossin)/1(cos
γγ
γγϕ
(6)
The power factor can be equal to unity due to the electronic converter placed between HSG and the load,
00
0000
cossin)1(
cossin
γγ
γγ
dqqBL
qdqds
XXXZZ
ZEXXE
V−−
−= (7)
If the HSG has a reduced saliency, then:
sBL
qds
sqd
XZZ
ZEE
V
XXX
0000 cossin γγ −=
=≅
(8)
It is clear that the d-axis excitation field acts in the same manner as the q-axis excitation field, therefore there is no reason to have both excitation windings. Without the q-axis excitation field the product between output HSG phase voltage and power factor comes as:
s
BL
ds
XZZ
ZEV 00 sincos γϕ = (9)
In an ISG case there is no need for an electronically controlled LC block since the voltage can be controlled only via d-axis variable field and via the electronic converter existing between the HSG and the load.
A general equation where the phase terminal voltage is given in function of load current IS, power factor, induced phase emf and HSG parameters comes after some mathematical developments as:
( ) ( )
22
022
sin)(
cossin
SqdqdSSS
qSqSS
IXXXXIVV
EXIXIV
+++=
=++
ϕ
ϕϕ (10)
Of course (10) would be simplified in the case of the ISG since the power factor is maintained almost equal to unity.
IV. HSM PROTOTYPE AND TESTS
A specific design procedure was developed based on existing literature, as [12, 13, 14]. Using that specific procedure and employing a lot of 2D FEM calculations, a HSM prototype was designed, constructed and tested [14]. The simplest solution was chosen, the permanent magnets (PMs) and the auxiliary winding on the interior rotor.
The cross section of the HSM prototype is shown in Fig. 7, and a detailed view of its rotor in Fig. 8.
The adopted solution has the advantage that many parts from a wound rotor induction motor could be used. The stator core was made a bit longer than that of the induction motor, the stator winding was changed accordingly to the calculation done for HSM and the rotor was entirely new constructed, but using the mechanical parts and the iron core steel sheets of the original induction motor. The main dimensions of the machine are given in Table II.
Fig. 7. HSM prototype cross section.
Fig. 8. HSM prototype rotor.
TABLE II HSM MAIN DIMENSIONS
Symbol Quantity Value
L Motor’s length 100 mm
Des Outer stator diameter 200 mm
Dis Inner stator diameter 124 mm
g Air-gap length 1 mm
hm Permanent magnet’s thickness 3.5 mm
θpm Permanent magnet angular width 72°
hss Stator slot height 21.8 mm
hrs Rotor slot height 17 mm
wrs Rotor slot width 22 mm
1198
The HSM parameters, obtained by tests or by 2D-FEM calculation are: − Stator phase resistance R = 1.7745 Ω, − Stator phase leakage inductance Lσ = 0.004975 H, − d-axis synchronous inductance Ld = 0.02028 H, − q-axis synchronous inductance Lq = 0.01586 H. The rated output power, when operating as a generator, is
5.16 kW at rated phase voltage Vs = 200 V and rated phase current Is = 8.6 A for unity power factor. Due to the imposed stator slot area the number of turns and the rated current had to be kept almost as for the original induction motor and consequently the prototype output voltage is much larger than the one required for an ISG. Anyway the prototype proved fully useful since it allowed for checking the design procedure and of all machine computed characteristics.
A lot of tests were performed on the prototype, some, concerning the generating regime being given here.
A comparison between phase emf values, 2D-FEM calculated, respectively test obtained, at no load is given in Fig. 9.
From the numerous terminal characteristics plotted based upon the computed, respectively measured results, here only two sets will be presented: those obtained at unitary power factor and Iex = 0 A, respectively Iex = 2.5 A, Fig. 10.
A very interesting characteristic, concerning the behaviour of the HSG in a large load domain is given in Fig. 11.
From the characteristic given in Fig. 11 one can see that at a very large variation of the load the output voltage can be maintained constant by varying the auxiliary field winding current. That is an important advantage of a HSM and it should be very carefully considered.
A similar characteristic can be obtained for a large speed domain, when the output voltage is maintained almost constant against the speed and load variation.
V. FIELD WEAKENING, HSM MOTORING REGIME
In the case of permanent magnet synchronous motors there is only one possibility to weaken the field, and, consequently to increase the speed domain, creating a d-axis stator flux opposite to the rotor field flux. In the case of SPM rotor synchronous motor the speed domain can be doubled, at maximum, through this method. It is quite un-sufficient for an ISG, besides the fact that it means additional losses produced in the stator winding and an important increase in the electronic converter current.
A comparison between the two ways of producing the flux weakening, via d-axis stator current and respectively additional field winding, can be made in the case of HSM.
Suppose that the length and the cross section of the stator and rotor winding are the same then R / Rf = N / Nf. In consequence the joule losses in flux-operating, with d-axis current injection and auxiliary excitation current, when the resulting air-gap flux is the same, are:
33
2
2
⎟⎟⎠
⎞⎜⎜⎝
⎛⋅=⇒⎟⎟
⎠
⎞⎜⎜⎝
⎛≅⋅=
NN
ppN
N
Ii
RR
pp f
jfjdf
f
d
fjf
jd (11)
It results, within these simplifying assumptions, which are not far of the reality, that the stator supplementary losses are much larger when the d-axis current is employed, for the flux weakening, than the auxiliary field winding losses at the same flux.
Thus, the efficiency is improved at the same speed gain,
-25 -20 -15 -10 -5 0 5 10 15 2040
60
80
100
120
140
160
180
200
220
240
260
Iex [A]
E [V
]
Field in oposition Field in the same direction
MeasuredComputed
Fig. 9. 2D-FEM and tests values, induced stator phase emf.
0 1 2 3 4 5 6 7 8 9170
175
180
185
190
195
200
205
210
I [A]
V s [V]
Iex = 0 A
Iex = 2,5 A
MeasuredComputed
Fig. 10. Output voltage plotted upon measured and computed data.
Fig. 11. HSG prototype excitation current vs. load current, phase voltage 190 V, unity power factor.
1199
since obviously the field winding number of turns is, or should be, larger than the stator phase number of turns. It can be seen from the simulations shown in Fig. 12, when the same speed increase was obtained.
In the following a flux weakening dynamic regime simulation, for the case when only the additional field current is employed, is presented.
The simulations were performed for a 5500 W, 1500 rpm, 48 Vcc sample HSM, with 12 poles, and a rated current of 81.3 A.
Its phase parameters are: the resistance 0.017 Ω; the d-q axis inductances 2.858·10-5 H, 2.142·10-5 H, respectively.
During the simulated dynamic regime the HSM was started, up to its rated speed, and after some time its speed was increased using flux weakening. The results of the simulation are given in Fig. 13 and they clearly evinced the speed increase due to the auxiliary field winding current presence.
As a matter of fact on the constructed prototype, within all the implicit limitations due to the use of an induction motor structure, the speed domain was extended more than three times, considering the base speed the one that corresponds to synchronous speed at 50 Hz.
VI. CONCLUSIONS
A HSM was presented as an alternative solution for ISG in hybrid and mild hybrid automobiles. Three HSM variants were analyzed. The best variant resulted that having both field sources, PMs and the additional field winding, on the rotor.
For this variant a prototype was designed, constructed and tested. The paper presents also some analytical developments and simulation results, beside the tests and the data concerning the prototype.
From all the developments and results presented a main conclusion is at hand: the HSM can be a very good choice for an ISG in hybrid cars since it has good power density, significant starting torque and large enough speed domain.
Moreover due to the possible field control in a generating operating mode, the output voltage can be largely controlled and therefore the electronic converter does not have to be designed for quite high voltages which could be generated at high speeds.
REFERENCES [1] I. Husain, "Electric and hybrid vehicles. Design fundamentals," CRC
Press LLC, Boca Raton (Fl, USA), 2003. [2] T. Lipman, R. Hwang, "Hybrid electric and fuel cell vehicle
technological innovation: hybrid and zero-emission vehicle technology links," Proc. of the 20th International Electric Vehicle Symposium and Exposition, Long Beach (CA, USA), 2003, pp. 111-119.
[3] M. Comanescu, A. Keyhani, M. Dai, "Design and analysis of 42-V permanent-magnet generator for automotive applications," IEEE Trans EC, vol. 18 (2003), no. 1, pp. 107-112.
[4] A.G. Jack, B.C. Mecrow, C. Weiner, "Switched reluctance and permanent magnet motors suitable for vehicle drives – a comparison, Proc. of the International Electric Machines and Drives Conference (IEEE IEMDC '99), Seattle (USA), 1999, pp. 345-350.
[5] I.-A Viorel, G. Henneberger, R Blissenbach, L. Löwenstein, "Transverse flux machines. Their behaviour, design, control and applications," Mediamira, Cluj (Romania), 2003.
[7] X. Luo, T.A. Lipo, "A synchronous permanent magnet hybrid ac machine," Proc. of the International Electric Machines and Drives Conference (IEEE IEMDC '99), Seattle (USA), 1999, pp. 19-21.
[6] C. Mi, "Rare-Earth Permanent Magnet Machines for Automotive Applications," Proc. of the 2nd Annual Winter Workshops Series, Vetronics Institute, College of Engineering and Computer Science, University of Michigan–Dearborn, Dearborn (MI, USA), 2002, pp. 234-241.
[8] D. Fodorean, I.A. Viorel, A. Miraoui, M. Gutman, "Comparison of hybrid excited synchronous motors for electrical vehicle propulsion," Proc. of Aegean Conferences on Electrical Machines and Power Electronics ACEMP '04, Istanbul (Turkey), 2004, pp. 52-57.
[9] S. Scridon, I. Boldea, L. Tutelea, F. Blaabjerg, A.E. Ritchie "BEGA – a biaxial excitation generator for automobiles: comprehensive characterization and test results," IEEE Trans. IA, vol. 41 (2005), no. 4, pp. 935-944.
[10] D. Fodorean, A. Djerdir, A. Miraoui, I.-A. Viorel, "Flux weakening performances for a double-excited machine" Proc. of ICEM '04, Krakow, Poland, 2004, paper no. 434 on CD-ROM.
[11] G. Henneberger and I.-A. Viorel, "Variable reluctance electrical machines," Aachen (Germany), Shaker Verlag, 2001.
[12] J.F. Gieras and M. Wing, "Permanent magnet motor technology, design and applications," New York Marcel Dekker, 1997.
[13] D.C. Hanselmann, "Brushless permanent magnet motor design," New York McGraw-Hill, 1994.
[14] G. Henneberger, J.R. Hadji-Minaglou, R.C. Ciorba, "Design and test of permanent magnet synchronous motor with auxiliary excitation winding for electric vehicle application" Proc. of EPE Chapter Symposium, pp. 645-649, Lausanne, 1994.
Fig. 12. Efficiency when HSM operates in a flux weakening regime (Id < 0 or If).
Fig. 13. The torque, speed and current in flux weakening with auxiliary field current If different to zero.
1200